With a recent advent of large-sized LCDs and a gradual expansion of their usage from portable devices, such as mobile phones, lap-top computers, etc., to home appliances, such as wall mounted flat panel TVs, there is a demand for LCDs with high definition and wide viewing angle. In particular, TFT-driven thin film transistor LCDs (TFT-LCDs) of which the individual pixels are independently driven are much superior in response speed of liquid crystals, realizing high-definition motion pictures, and thus increasingly used in a wider range of applications.
To be used as an optical switch in the TFT-LCDs, liquid crystals are required to initially align in a defined direction on a layer including innermost TFT of the display cell. For this, a liquid crystal alignment layer is used.
For the liquid crystal alignment to occur, a heat-resistant polymer such as polyimide is applied on a transparent glass to form a polymer alignment layer, which is then subjected to a rubbing process using a rotary roller wound with a rubbing cloth of nylon or rayon fabrics at a high rotation speed to align liquid crystals.
However, the rubbing process remains mechanical scratches on the surface of the liquid crystal alignment layer or generates strong static electricity, possibly destroying the TFTs. Moreover, fine fibers coming from the rubbing cloth may cause defectives, imposing an obstacle to acquiring a higher production yield.
To overcome the problems with the rubbing process and achieve innovation in the aspect of production yield, there has been derived a liquid crystal alignment (hereinafter, referred to as “photo-alignment”) method using a light such as UV radiation.
Photo-alignment refers to the mechanism using a linearly polarized UV radiation to cause the photoreactive groups of a defined photoreactive polymer to participate in a photoreaction, ending up with the main chain of the polymer aligned in a defined direction to form a photo-polymerized liquid crystal alignment layer with aligned liquid crystals.
The representative example of the photo-alignment is photopolymerization-based photo-alignment as disclosed by M. Schadt et al. (Jpn. J. Appl. Phys., Vol 31, 1992, 2155), Dae S. Kang et al. (U.S. Pat. No. 5,464,669), and Yuriy Reznikov (Jpn. J. Appl. Phys. Vol. 34, 1995, L1000). The photo-aligned polymers used in these patent and research papers are mostly polycinnamate-based polymers, such as poly(vinylcinnamate) (PVCN) or poly(vinyl methoxycinnamate) (PVMC). For photo-alignment of polymers, the double bond of cinnamate exposed to UV radiation participates in a [2+2] cycloaddition reaction to form cyclobutane, which provides anisotropy to cause liquid crystal molecules aligned in one direction, inducing liquid crystal alignment.
Besides, JP11-181127 discloses a polymer and an alignment layer including the same in which the polymer has a side chain including photoreactive groups such as cinnamate on a main chain such as acrylate, methacrylate, etc. Korean Patent Laid-Open Publication No. 2002-0006819 also discloses the use of an alignment layer comprising a polymethacryl-based polymer.
Such conventional photoreactive polymers for alignment layer, however, have their polymer main chain with low thermal stability and thus undesirably contribute to deterioration in the stability of the alignment layer or poor characteristics regarding photoreactivity, liquid crystal alignment, or alignment rate. For example, polymers with an acryl-based main chain have low thermal stability, which contributes to a great deterioration in the stability of the alignment layer, while polymers with photosensitive groups belonging in the main chain cannot rapidly react to the polarized light irradiated on the alignment layer and thus deteriorate liquid crystal alignment or alignment rate. Such deterioration in liquid crystal alignment or alignment rate causes reduced process efficiency or contributes to insufficient liquid crystal alignment of the LCDs, leading to lower dichroic ratio and poor contrast.
On the other hand, Bull. Korean Chem. Soc. 2002, Vol. 23, 957 discloses a photoreactive polymer containing a repeating unit bound to an azo group as a part of the repeating units. However, such a photoreactive polymer also has poor alignment and low photoreaction rate and still results in low process efficiency and poor contrast of the LCDs.